The purpose of box squats is to break the concentric/eccentric chain. This builds your explosive power out of the hole quite well. Vary the box heights each week, use 2-3 different heights (1-2" above, 1-2" below, and 3"+ below parallel). I like to work from low to high and then take a few weeks to focus on front squats and such, but the westsiders use the box squat every week (just with different stances, bars, or boxes).
I let the tension out of my legs completely but keep my back very straight while sitting on the box. 1-2secs and back up.